1. Field of the Invention
The present invention is directed to a zirconia based ceramic material having excellent mechanical strength and toughness, and also a process of making the same product.
2. Disclosure of the Prior Art
A partially stabilized zirconia, which is obtained by the addition of a preferable amount of a stabilizer such as yttrium oxide (Y.sub.2 O.sub.3), cerium oxide (CeO.sub.2), magnesium oxide (MgO), or the like., to zirconia (ZrO.sub.2), has been well known as a ceramic material having higher mechanical strength and toughness than another popular ceramic materials such as alumina (Al.sub.2 O.sub.3), silicon carbide (SiC), silicon nitride (Si.sub.3 N.sub.4), or the like. The partially stabilized zirconia possesses a unique mechanism for improving the mechanical strength and toughness. That is, a phase transformation from metastable tetragonal zirconia to stable monoclinic zirconia is caused under a stress concentration, for example, at the tip of a crack developed in the partially stabilized zirconia. Since such a transformation, which is usually called as a stress induced phase transformation, is accompanied with a volume expansion of about 4%, a further progress of the crack in the partially stabilized zirconia can be effectively prevented. For example, a partially stabilized zirconia containing Y.sub.2 O.sub.3 as the stabilizer exhibits a mechanical strength of 1000 MPa or more. Particularly, it is already known that a ceramic material comprising Al.sub.2 O.sub.3 grains as a dispersing phase and the partially stabilized zirconia containing Y.sub.2 O.sub.3 as a matrix exhibits a higher mechanical strength of 2400 to 3000 MPa. However, these ceramic materials provide a relatively low mechanical toughness which is in a range of 5 to 6 MPa m.sup.1/2 and not always enough to use as structural and machine parts. On the other hand, a partially stabilized zirconia containing CeO.sub.2 as the stabilizer exhibits a high mechanical toughness of 10 to 20 MPa m.sup.1/2. Additionally, the partially stabilized zirconia containing CeO.sub.2 is capable of stably maintaining the metastable tetragonal zirconia at a temperature range of 200.degree. C. to 400.degree. C., and particularly keeping the mechanical strength even under a hydrothermal condition. The partially stabilized zirconia containing Y.sub.2 O.sub.3 exhibits a low temperature degradation of mechanical strength at the temperature range of 200.degree. C. to 400.degree. C. When the partially stabilized zirconia containing Y.sub.2 O.sub.3 is kept at the temperature range for an extended time period, most of tetragonal zirconia included in the partially stabilized zirconia containing Y.sub.2 O.sub.3 is transformed to monoclinic zirconia accompanying with the volume expansion of about 4%. By the volume expansion, micro cracks are developed in the partially stabilized zirconia containing Y.sub.2 O.sub.3, so that the mechanical strength is decreased. In addition, the tetragonal-to monoclinic phase transformation is accelerated under the hydrothermal condition, so that the mechanical strength is considerably decreased. Thus, the partially stabilized zirconia containing CeO.sub.2 exhibits a higher mechanical strength than the partially stabilized zirconia containing Y.sub.2 O.sub.3 at the above temperature range, and particularly under the hydrothermal condition. However, the mechanical strength of the partially stabilized zirconia containing CeO.sub.2 is in a range of 600 to 800 MPa at a room temperature, which is lower than that of the partially stabilized zirconia containing Y.sub.2 O.sub.3. Therefore, by improving mechanical strength of the partially stabilized zirconia containing CeO.sub.2, more expanded application fields will be expected as a material overcoming the low temperature degradation of the mechanical strength of the widely-used partially stabilized zirconia containing Y.sub.2 O.sub.3. In other words, a zirconia based ceramic material having improved and well-balanced mechanical strength and toughness is desired.
Japanese Early Patent Publication KOKAI! No. 5-246760 discloses a zirconia based composite material comprising a matrix of a partially stabilized zirconia containing 5 to 30 mol % of CeO.sub.2 and a secondary phase of at least one selected from Al.sub.2 O.sub.3, SiC, Si.sub.3 N.sub.4, B.sub.4 C, carbides, nitrides and borides of elements of groups IVa, Va, VIa of the periodic table. Fine grains of the secondary phase are dispersed within grains as well as in grain boundaries of the partially stabilized zirconia matrix. When the content of CeO.sub.2 is more than 30 mol %, the mechanical strength of the composite material is lowered due to the increase of cubic zirconia in the matrix. When the content of CeO.sub.2 is less than 5 mol %, a formation of metastable tetragonal zirconia in the matrix is not enough. The composite material contains 0.5 to 50 vol % and more preferably 2.5 to 30 vol % of the secondary phase.
Japanese Early Patent Publication KOKAI! No. 63-144167 discloses the following ceramic materials (a) to (c):
(a) a ceramic material comprising 45-94.75 mol % of ZrO.sub.2, 5-45 mol % of TiO.sub.2 and 0.25-10 mol % of a rare earth oxide selected from the group consisting of Gd.sub.2 O.sub.3, Yb.sub.2 O.sub.3, Nd.sub.2 O.sub.3, Tb.sub.2 O.sub.3, Pr.sub.2 O.sub.3, Dy.sub.2 O.sub.3, Ho.sub.2 O.sub.3, Sm.sub.2 O.sub.3, and Er.sub.2 O.sub.3 ;
(b) a ceramic material comprising 48-94.75 mol % of ZrO.sub.2, 5-45 mol % of TiO.sub.2 and 0.25-7 mol % of Y.sub.2 O.sub.3 ;
(c) a ceramic material comprising 35-93.5 mol % of ZrO.sub.2, 5-45 mol % of TiO.sub.2 and 1.5-20 mol % of CeO.sub.2.
For example, in the ceramic material (a), a grain growth of tetragonal zirconia is prevented by adding a small amount of the rare earth oxide to a partially stabilized zirconia containing TiO.sub.2 as the stabilizer, so that the mechanical properties are improved. In addition, this prior art discloses that grains of cubic zirconia and/or zirconium titanate (ZrTiO.sub.4) are effective to prevent the grain growth of tetragonal zirconia. These ceramic materials (a) to (c) can be used as a dispersing phase to reinforce a ceramic matrix such as alumina, mullite, or the like. For example, a ceramic composite consisting of about 5 vol % of the ceramic material and the balance of the ceramic matrix may be useful as a material improving mechanical properties of the ceramic matrix.
Japanese Patent Publication KOKOKU! No. 64-7029 discloses a ceramic material comprising 61 to 87 wt % of ZrO.sub.2, 11 to 27 wt % of CeO.sub.2 and 20 wt % or less of Al.sub.2 O.sub.3. The ceramic material mainly contains tetragonal zirconia, in which CeO.sub.2 forms a solid solution with ZrO.sub.2. 20% or less of the entire zirconia crystals in the ceramic material is monoclinic zirconia and/or cubic zirconia. This prior art discloses that a sintered body produced by preparing a mixture of ZrO.sub.2, CeO.sub.2 and Al.sub.2 O.sub.3 with the above composition and sintering the mixture exhibits a mechanical strength comparable to a sintered body in ZrO.sub.2 -Y.sub.2 O.sub.3 system. This prior art also indicates that the ceramic material may contains 2 wt % or less of SiO.sub.2, TiO.sub.2 and Fe.sub.2 O.sub.3, etc., as impurities. That is, since a ZrO.sub.2 powder used in this prior art is prepared by treating a zircon sand containing the impurities with an acid to obtain an acid chloride of zirconium, and performing a heat-treatment to the acid chloride, there is a probability of remaining a small amount of the impurities in the ZrO.sub.2 powder. On the other hand, when the content of Al.sub.2 O.sub.3 is more than 20 wt %, a sintering temperature of the ceramic material is raised, so that the grain growth of the zirconia crystals is enhanced. By such a grain growth, variations in mechanical strength of the ceramic material are increased.
Thus, various attempts have been made to improve the mechanical properties of the zirconia based ceramic material.